How do you cook a turkey on a gas grill?

I have a large 21 inch gas grill (Ducane) and I want to cook my turkey (about 18 lbs) on it. I do have a spit attachment. Can I do this? How do I do this? Thank You! I am going to use the Judy Bird recipe and dry brine.

Cook's Illustrated has directions for both charcoal and gas grilled turkey, Nov/Dec 1999. Naturally, they have all the science behind both methods. We tried the charcoal one a few Thanksgivings ago and it was excellent. We grill turkey thighs often (especially excellent for traditional Mole Poblano). If you do not have a friend with a back stash of Cook's, you can subscribe to their internet service for a small fee for a couple of weeks & print out as much as you want. Worth it.

The turkey turned out perfectly! I highly recommend this method. It gave us the ability to use the oven for other things. Also, the 15 lbs turkey cooked in under 2 hours. The only method is to watch the flame to make sure the temp is consistent-- but that wasn't too hard.